During the last few years, several studies have evaluated the health risks associated with exposure to engine exhaust emissions. As a result of these studies, increasing government and health organizations have decided to tighten the standards which apply to engine-run vehicles, their fuels and their particulate and gaseous emissions.
On Nov. 15, 1990, the President of the United States signed into law the Clean Air Act Amendments of 1990. Beginning in 1994, the new law sets a performance criteria, particularly requiring buses operating more than 70 percent of the time in large urban areas (using any fuel) to cut particulates by 50 percent compared to conventional heavy duty vehicles. Also, beginning in 1994, the Environmental Protection Agency began requiring a yearly testing to determine whether buses subject to the standard are meeting the standard in use over their full useful life. Similar provisions exist in other countries and a global effort is underway to find exhaust filters and cleaning devices. In this regard, several countries in the first and third world have been actively cooperating to reduce emissions from exhaust systems. Essentially, this provision allows the use of exhaust after treatment devices to reduce particulate emissions to a very low level provided that they work in the field.
Because of the financial and logistical concerns with alternative fuels, transit authorities and bus engine manufacturers are seriously considering after treatment systems such as trap-oxidizer technology to meet 1993/94 EPA laws and regulations. Bus engines, for example, run on a stop-and-go cycle which forces the engines to operate with a dirty and sooty exhaust. Second, these vehicles operate in dense population areas and hence, bus exhaust and pollution is considered a greater health hazard than over-the-road trucks. Third, environmentalists would like to be as clean as possible even if it means going beyond EPA regulations. All of these factors make trap oxidizer technology very attractive, provided that its long-term durability can be proven and made available at reasonable costs.
The 1993/94 EPA law and regulations are only the first step in a series of ever-tightening regulations to follow. For the diesel engines industry, the next step in regulation occurs in 1998, when the laws require tighter NO.sub.x control. Even though NO.sub.x reduction for 1994 levels will be achieved by improved engine design, it is generally accepted that to meet the 1998 levels of 4 g/Bhp-h NO.sub.x, diesel engines will have to use after treatment systems. As the NO.sub.x level is reduced, however, the particulate level increases. Hence, in trying to meet the 1998 low NO.sub.x levels, engine manufacturers are faced with increased particulates, which require further use of aftertreatment devices such as converters and traps.
The coated devices of the present invention provide improved regenerative systems which offer the flexibility, efficiency, and cost-effectiveness needed to meet the challenges presented by near-term (1994) and medium-term (1998) diesel emissions regulations.
The filters used in aftertreatment trap systems are the core of the system and great efforts are being made to fine-tune the existing systems to improve their effectiveness and durability. One of the problems of the current technology is in "regenerating" the filter by burning off the accumulated particulate matter. Initiating and controlling the regeneration process to ensure reliable regeneration without damage to the trap is the central engineering problem of trap oxidizer development today. The reason is that over time, the filter becomes loaded with the soot it has trapped and must be cleaned or "regenerated". The process of regeneration burns or "oxidizes" the soot collected within the filter. The cleaned filter can be used many times provided it can be successfully regenerated many thousands of times over its lifetime without failure. Many different regeneration concepts are being tested. They range from primitive off-board regeneration of the filter in an external oven to sophisticated on-board automatic electrical or burner regeneration systems using electronic controls and include catalytic injection systems. These approaches to regeneration can generally be divided into two groups: passive systems and active systems. Passive systems must attain the conditions required for regeneration during normal operation of the vehicle. Active systems, on the other hand, monitor the build up of particulate matter in the trap and trigger specific actions leading to regeneration when needed.
Passive regeneration systems face special problems on heavy duty vehicles. Exhaust temperatures from heavy duty diesel engines are normally low, and recent developments such as charge air cooling and increased turbo charger efficiency are reducing them still further. Under some conditions, it would be possible for a truck driver to drive for many hours without exceeding the exhaust temperature (around 400.degree.-450.degree. C.) required to trigger regeneration.
Active systems, on the other hand, are generally expensive, often requiring complex logic and electronics to initiate regeneration.
Engine and catalysts manufacturers have experimented with many catalytic converters and with a wide variety of regenerative catalytic traps, Precious metal catalytic traps are effective in oxidizing gaseous hydrocarbons and CO, but are relatively ineffective in promoting soot oxidation, a particular problem for diesel engines. Moreover, these metals also promote the oxidation of SO.sub.2 to particulate sulfates such as sulfuric acid (H.sub.2 SO.sub.4), Base metal catalytic traps, in contrast, are effective in promoting soot oxidation, but have little effect on hydrocarbons, CO, NO or SO.sub.2. Another disadvantage of precious metal catalysts is that they are very expensive.
Unlike a catalytic trap, however, a flowthrough catalytic converter does not collect most of the solid particulate matter, which simply passes through in the exhaust. The particulate control efficiency of the catalytic converter is, of course, much less than that of a trap. One of the major disadvantages of the catalytic converter is the same as with the precious metal catalytic particulate trap: sulfate emissions. The main object of the catalysts used is to raise the exhaust temperature to a point that could convert the gaseous compounds to safer gaseous emissions. The catalysts undergo chemical reactions which raise the temperature of the exhaust gases allowing them to be converted to the safer gases. One of the major reasons which catalytic material and treatments are used to assist in trap regeneration, is that none of the heating systems attempted, such as diesel fuel burners, electrical heaters and other heaters have been successful. However, if there were a regeneration system in which a converter or trap could be used without a catalyst for regeneration, the above-listed objects would be achieved.
With respect to processes for the manufacture of porous ceramic articles, U.S. Pat. No. 3,090,094, issued May 21, 1963 to K. Schwartzwalder et al, discloses a method of making an open-cell porous ceramic article which comprises immersing an open-cell spongy material, preferably polyurethane, in a slurry containing a ceramic coating material to coat cell-defining walls of the spongy material, removing excess slurry from the spongy material, and firing the coated spongy material at a temperature and for a time sufficient to remove the spongy material and form a hardened, vitrified structure. The ceramic coating material may include particulate zirconia, zircon, petalite, mullite, talc, silica and alumina, having particle sizes ranging from -80 mesh to -600 mesh. A binder such as clay, sodium silicate, and calcium aluminate and phosphoric acid, is preferably present in the slurry. Firing is conducted at 500.degree. to 3000 .degree. F. (260 .degree. to 1650 .degree. C.), preferably at 2100 .degree. to 2950 .degree. F. (1150.degree. to 1620.degree. C.).
U.S. Pat. No. 3,097,930, issued Jul. 16, 1963 to I. J. Holland, discloses a method of making a porous shape of sintered refractory material which comprises impregnating a foamed plastic sponge shape with a suspension of refractory particles, drying the impregnated shape, and firing the dried shape in an inert atmosphere to volatilize the sponge material and to sinter the refractory particles. The impregnation and drying steps may be repeated. The foamed plastic sponge may be polystyrene, polyethylene, polyvinyl chloride, latex, or polyurethane, the latter being preferred. Refractory materials include clays, minerals, oxides, borides, carbides, silicides, nitrides and mixtures thereof. Specific examples used alumina, beryllia and china clay with particle sizes ranging from less than 1 to greater than 10 microns. Firing was conducted at 1700 .degree. C. for alumina and 1350 .degree. C. for china clay.
U.S. Pat. No. 4,697,632, issued Oct. 6, 1987 to N. G. Lirones, discloses a ceramic foam filter, insulating refractory lining, and a melting crucible, and a process for production thereof, which comprises providing an open-cell foam pattern, impregnating the pattern with a ceramic slurry, burning out the foam pattern at a temperature between 1400 .degree. and 2200 .degree. F. (760 .degree. and 1205 .degree. C.) to form a ceramic substrate, impregnating the ceramic substrate with additional ceramic slurry, and firing the impregnated ceramic substrate at a temperature of 2200 .degree. to 3400 .degree. F. (1205 .degree. to 1870 .degree. C.). The foam pattern material may be a flexible polyurethane, polyethylene, polypropylene or graphite. A suitable ceramic slurry contains from 1% to 20% silica (dry weight), and from 99% to 80% alumina (dry weight), with a viscosity between 5 and 20 seconds and a film weight between 1.0 and 8.0 grams per standard six inch square plate. Preferably the slurry includes a suspending agent, a wetting agent and a defoaming agent. Zirconia may also be used as ceramic material.
U.S. Pat. No. 3,111,396, issued Nov. 19, 1963 to B. B. Ball, discloses a method of making a porous metallic article which comprises impregnating a porous organic structure with a suspension of powdered metal, metal alloy or metal compound, and binder, slowly drying the impregnated structure, heating at about 300.degree.-500.degree. F. (150.degree.-260.degree. C.) to char the organic structure, and then heating at about 1900.degree. to about 3000.degree. F. ( 1040.degree. to 1650.degree. C.) to sinter the powder into a porous material.
Other United States patents relating to porous ceramic filters and methods for making them include: 3,893,917--Jul. 8, 1975--M. J. Pryor et al; 3,947,363--Mar. 30, 1976--M. J. Pryor et al; 3,962,081--Jun. 8, 1976--J. C. Yarwood et al; 4,024,056--May 17, 1977--J. C. Yarwood et al; 4,081,371--Mar. 28, 1978--J. C. Yarwood et al; 4,257,810--Mar. 24, 1981--T. Narumiya; 4,258,099--Mar. 24, 1981--T. Narumiya; and 4,391,918--Jul. 5, 1983--J. W. Brockmeyer.
None of the above patents disclose or suggest the desirability of using conductive filters, which can also be used as heating elements. Additionally, there is no suggestion in any of the above patents to impregnate a substrate with a ceramic or ceramic composite slurry in the manner undertaken by the present invention. The problems associated with the prior art methods are similar to the problems associated with the method described in U.S. Pat. No. 5,279,737, which problems are described in greater detail below.
U.S. Pat. No. 5,279,737 ("the '737 patent") discloses a process for producing a porous ceramic, ceramic composite or metal-ceramic structure by micropyretic synthesis wherein a form polymer shape is impregnated with a slurry of ceramic precursors and ignited to initiate micropyretic synthesis, thereby attaining a ceramic, ceramic composite or metal-ceramic composite article having interconnected porosity. The '737 patent is incorporated by reference into the present application, in its entirety. "Micropyretics" or "micropyretically synthesized," as used herein refers to self propagating high temperature synthesis as discussed in the review article by Subrahmanyam et al., in The Journal of Micromolecular Science at Vol. 27, p.p. 6249-6273.
As will now be described, the present process also constitutes a novel and unobvious improvement over the process described in the '737 patent. The impregnation step in the '737 patent is achieved by dipping the polymeric foam in the slurry with which it is to be impregnated. This step is very cumbersome and awkward. Also during processing using the invention of the '737 patent, one has be extremely careful so that the "green structure" (the structure before sintering, micropyretic or otherwise), does not "collapse." Collapse as used herein refers to dissolution of the ceramic in structure, before sintering, before or after burning of the polymeric foam). The process of the '737 patent may also give rise to "distortion." Distortion as used herein means physical distortion which results from large structure sagging under its own weight prior to burning of the polymeric foam. The impregnation of the present process is achieved by (a) fluidizing said slurry with steam and spraying the shape with said fluidized slurry or (b) heating said slurry so as to reduce its viscosity and spraying the shape with said reduced viscosity slurry. This method of impregnation eliminates the above listed problems. Additionally, the steam or hot liquid constituent of the spray better dissolves certain constituents, such as calcium carbonates and silicates (cements) such as (CaO).sub.3.(SiO.sub.2).sub.2.(H.sub.2 O).sub.3, which lead to a high green and final strength by precipitating out on the deposited surface as a cement. The present process also results in more uniform thickness of the ceramic.
The patent application of which this application is a continuation-in-part application, U.S. Ser. No. 08/353,727 filed Dec. 12, 1994, ("the '727 application") discloses a modulated filter for gaseous, liquid and particulate matter wherein the modules in said filter are porous ceramic or ceramic composite structures, said structures having interconnected porosity and having been manufactured using micropyretic synthesis, the filter comprising at least two porous ceramic or ceramic composite modules. Preferably each said module is optimized for extracting different materials. The '727 application also discloses a regenerator filter comprising a means for filtering and a means for regenerating said means for filtering, said means for regenerating being integral with said filtering means. The second aspect of the '727 application is of greater relevance to the present case. The '727 application is hereby incorporated by reference herein, in its entirety.
The regenerative filters of the '727 application comprise a heating element and a porous filter. However, there is no suggestion or motivation to one skilled in the art to modify the porous filters of the '727 application to make them conductive. There is also no suggestion of the desirability of making filters conductive so that the filters can simultaneously act as heating elements, thereby removing the need for a heating element. Additionally, there is no suggestion of applying any type of coating to the ceramic filters. Furthermore, the '727 application is devoid of any suggestion of using non-polymeric starting materials. Furthermore, because the heating element and filter are one and the same, less heat is lost due to radiation and the heat is available precisely where required, i.e. in the filter (which is also the heating element). The present application, on the other hand, achieves all the above listed properties and is therefore novel and unobvious over the '727 application.